1. Field of the Invention
This invention relates to a burner having reduced NO.sub.x emissions and, in particular, to a burner wherein flow and mix rates may be varied in accordance with the combustion characteristics and demand rate of the burner. The specific adjustments of an existing burner may be retrofitted to vary for optimization with demand.
2. Description of the Prior Art
Combustion system burners have come under increased scrutiny for the toxic emissions which are a by-product of the combustion process. Depending upon the extent of combustion, carbon monoxide and NO.sub.x may be omitted at unacceptable levels. Carbon monoxide levels can normally be controlled through complete combustion resulting in carbon dioxide. However, three factors contribute to the formation of NO.sub.x in combustion systems. The first and most widely recognized is flame temperature. Most current systems incorporate some method of staging fuel and air to reduce flame concentration and resultant high temperatures. A second factor is excess O.sub.2 levels. Higher O.sub.2 levels tend to provide more oxygen for combination with nitrogen; however, the higher O.sub.2 levels results in excess air which tends to balance the effect of lower temperatures. The laminar mix in most current low NO.sub.x burners requires more O.sub.2 for complete combustion. If lower O.sub.2 levels are utilized the result is incomplete combustion in the form of carbon monoxide. The third factor is residence time in a critical temperature zone which is virtually ignored in modern burners because reduced time means higher velocities producing unacceptable temperatures.
One common practice for reducing NO.sub.x levels is to use external, induced or forced flue gas recirculation (FGR). A common misconception about FGR is that the process is destroying NO.sub.x in the original flue gas. However, recent research has determined that FGR simply reduces or dilutes the flame front thereby reducing the formation of NO.sub.x. Further, external flue gas recirculation results in higher temperature and increased volume combustion air producing higher pressure drops through the system requiring more horsepower, the resultant higher velocities also reducing heat transfer thereby reducing the efficiency of the burner.
Several burner manufacturers have developed low NO.sub.x systems with mixed results. Although NO.sub.x systems emissions have been reduced many of the systems do not meet the stringent emission levels. Moreover, the modern burners are specifically designed for the particular application and will not control emissions in different combustion systems or under different conditions because of their inflexibility. An additional drawback in prior known systems, as NO.sub.x emissions were reduced the carbon monoxide (CO) levels would increase.